The story is based loosely on Shakespeare's "Romeo and Juliet." "R" = "Romeo;" "Julie" = "Juliet; "Perry" = "Paris;. "M"/"Marcus" = "Mercutio;" "Nora" = Juliet's "Nurse" (the character of Nora is also a nurse).
When R returns to the airport with Julie for the first time after the raid at the lab, an automated announcement in the background can be heard, saying, "The white zone is for loading and unloading of passengers only." This is taken from Airplane! (1980) at the beginning of the movie, when Ted first arrives at the airport.
The same car is seen in the same angle three times, but each time, the car gets reconstructed a little bit more (the first time right after Julie escapes in the car and R walks in the sun, then when he walks alone in the rain, and the last time after he meets with the group of zombies walking at the sound of "Rock You like a Hurricane").
The trivia items below may give away important plot points.
The actors portraying the zombies in this film chose not to blink. It was particularly uncomfortable for Nicholas Hoult, who did a few long scenes without blinking. Later in the film (when the zombies become more human), the zombies start blinking.
The scene where R and Julie jump off the ledge into the water seems to portray the tragic ending of the play where both Romeo and Juliet die in each other's arms. However their survival and R's recovery provide the twist to make this tragic play a romantic comedy movie.